Whereas the nostrils of old world monkeys always turn downwards and their tails are never prehensile, in new world monkeys nostrils face sideways and most species have tails that can grasp objects.
A. Whereas the nostrils of old world monkeys always turn downwards and their tails are never prehensile, in new world monkeys nostrils face sideways -> nostrils are compared with monkeys. Incorrect.
B. Unlike old world monkeys, having nostrils that always turn downwards and tails that are never prehensile, the nostrils of new world monkeys face sidewards -> monkeys are compared with nostrils. Incorrect.
C. Unlike old world monkeys, in whom nostrils always turn downwards and tails are never prehensile, new world monkeys have nostrils that face sideways -> Comparison is correct. But, why do we say "nostrils that face sideways", we could have said "nostrils face sideways". Incorrect.
D. Unlike the downward-facing nostrils and non-prehensile tails of old world monkeys, in new world monkeys nostrils are sideward-facing -> Similar as A. Incorrect.
E. Whereas in old world monkeys nostrils always turn downwards and tails are never prehensile, in new world monkeys nostrils face sideways -> This is better. Let's keep it.
So, I think E.